Consider the blue car: To make a left turn, you have to make a right turn first.

A throughabout

A throughabout (also known as “hamburger roundabout” or “cut-through” – http://en.wikipedia.org/wiki/Roundabout#Hamburger_roundabout) is a type of road junction that connects exactly one major road with one or more minor roads using a circled traffic. The major road is the one that passes through the circle traffic. Coming from the major road, you may not make a direct left turn. Instead, you have first to make a right-turn to leave the major road and enter in the circled traffic

Throughabouts can be quite small. In this case, usually they are not signal controlled; the mayor road has the right of way. However, they can also be quite big and have a lot of traffic signals.

In spite of their name, throughabout/hamburger roundabouts are not roundabouts. (No priority for the traffic in the roundabout and no non-traversable island). In this situation the tag junction=roundabout has to be avoided!

How to tag (major road with single carrigeway)

Circle traffic

Tagging with oneway=yes and highway=*. Do not use junction=roundabout on the circle traffic!